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Before we go into possible theories

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as to why plates actually move,

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what I want to do in this video is

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think a little bit about why we see the

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geological features we do see

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at plate boundaries, and in particular

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I want to focus on the

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features we see at divergent plate boundaries,

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where the plates are moving away

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from each other, where new land is being created

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like we saw in the mid oceanic ridges,

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where we see new land being created right in the center

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and moving outwards from them.

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So to do that let's think about the different layers

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and actually I want to make one quick correction

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on the last video.

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Over here I had drawn these arrows going in that direction

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and based on how I defined them, they should

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be going into the page and so they should've

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had these Xs here.

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Now with that out of the way

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let's draw a little diagram

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of what happens at the early stages

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of these divergent plate boundaries.

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So you might have your

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here you have just your crust,

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and maybe it's continental crust...

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This right here is the earth's crust,

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and then you have the solid part of the mantle.

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And the combination of them is the lithosphere.

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And then you have the liquid part, the superhot part

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of the mantle, this down here is magma.

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It hasn't solidified, it's hot enough to be in the liquid state.

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And all of this combined, this right here,

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we consider the mantle.

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Now, there's some debate, and we'll talk about

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this in the next video, of how hotspots actually form

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it could be these mantle plumes that start

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at the border between the mantle and the core,

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it could be some type of convection current

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in the actual mantle, we'll talk more about that in

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the next video or maybe a few videos from now.

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But let's take it for granted that hotspots form

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in the mantle, so let's say we have an area

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of magma right here, that is particularly hot.

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Let me draw it in in another color... pink.

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So this is particularly hot magma here.

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And we know, or maybe we don't know,

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if you take the same material and you make it hotter,

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it's going to become less dense

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because the particles essentially are going to

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bump into each other with more kinetic energy

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and have more space in between them

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and so this really hot part of the magma

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or this really hot part of the mantle

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it is going to move upwards,

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because it is less dense, it will have buoyancy.

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And as it moves upwards, it'll heat up

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the things around it and it'll

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eventually make its way into the lithosphere

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and it'll be able to break through the lithosphere

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because it's so hot it can melt its way through.

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So let's fast forward this a little bit,

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so this is Step 1 up here.

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Now Step 2, this hot magma is